Blood coagulation is a complex chemical and physical reaction which occurs when the blood comes into contact with an activating agent, such as an activating surface or an activating reagent. In accordance with one simplified conceptual view, the whole blood coagulation process can be generally viewed as three activities: agglutination of platelets, blood clotting and fibrous tissue formation. In vivo, platelets flow through the blood vessels in an inactivated state because the blood vessel lining, the endothelium, does not activate platelets. When a blood vessel is damaged, however, the endothelium loses its inert character and activates the platelets at the damage site. Activation of the platelets causes them to become "sticky" and adhere together. Additional platelets then adhere to the activated platelets and also become activated. This process continues until a platelet "plug" is formed. This platelet plug then serves as a matrix upon which blood clotting proceeds.
If the chemical balance of the blood is suitable, thrombin is then produced which causes polymerization of fibrinogen into fibrin strands, which form the major portion of the clot mass. During clotting, additional platelets are activated and trapped in the forming clot, contributing to clot formation. As clotting proceeds, fibrous tissue is formed which serves as the permanent clot. Thus platelet activation plays a very important function in blood coagulation.
A number of different medical apparatus and testing methods exist for measuring and determining coagulation and coagulation related activities of blood. These apparatus and methods provide valuable medical information to an attending physician. For example, the information assists a physician in prescribing medication, predicting post-operative bleeding and prescribing various therapies. One of the more successful techniques of evaluating blood clotting and coagulation is a plunger technique illustrated by U.S. Pat. Nos. 4,599,219 to Cooper et al. and 4,752,449 to Jackson et al., and U.S. patent application No. 07/644,007 to Smith, filed Jan. 18, 1991, now U.S. Pat. No. 5,174,961, all of which are assigned to the assignee of the present invention, and all of which are incorporated herein by reference.
Automated apparatus employing the plunger technique for measuring and detecting coagulation and coagulation related activities generally comprise a plunger sensor cartridge or cartridges and a microprocessor controlled apparatus into which the cartridge is inserted. The apparatus acts upon the cartridge and the blood sample placed therein to induce and detect the coagulation related event. The cartridge includes a plurality of test cells, each of which is defined by a tube-like member having an upper reaction chamber where a plunger assembly is located and where the analytical test is carried out, and a lower reagent chamber which contains a reagent or reagents. For an activated clotting time (ACT) test, for example, the reagents include an activation reagent to activate coagulation of the blood. A plug member seals the bottom of a lower reagent chamber. When the test commences the contents of the lower reagent chamber are forced into the reaction chamber to be mixed with the sample of fluid, usually human blood or its components. An actuator, which is a part of the apparatus, lifts the plunger assembly and lowers it, thereby reciprocating the plunger assembly through the pool of fluid in the reaction chamber. The plunger assembly descends by the force of gravity, resisted by a property of the fluid in the reaction chamber, such as its viscosity. When the property of the sample changes in a predetermined manner as a result of the onset or occurrence of a coagulation related activity, the descent rate of the plunger assembly therethrough is changed. Upon a sufficient change in the descent rate, the coagulation related activity is detected and indicated by the apparatus.